Method of Separating Light-Emitting Diode from a Growth Substrate

ABSTRACT

A method of forming a light-emitting diode (LED) device and separating the LED device from a growth substrate is provided. The LED device is formed by forming an LED structure over a growth substrate. The method includes forming and patterning a mask layer on the growth substrate. A first contact layer is formed over the patterned mask layer with an air bridge between the first contact layer and the patterned mask layer. The first contact layer may be a contact layer of the LED structure. After the formation of the LED structure, the growth substrate is detached from the LED structure along the air bridge.

This application claims priority to U.S. application Ser. No.12/554,578, filed on Sep. 4, 2009, entitled “METHOD OF SEPARATINGLIGHT-EMITTING DIODE FROM A GROWTH SUBSTRATE,” which claims the benefitof U.S. Provisional Application No. 61/147,677, filed on Jan. 27, 2009,entitled “Method of Separating Light-Emitting Diode from a GrowthSubstrate”, and U.S. Provisional Application No. 61/095,773, filed onSep. 10, 2008, entitled “Method of Separating Light-Emitting Diode froma Growth Substrate”, the entire disclosures of each of which are herebyincorporated herein by reference in their entirety.

TECHNICAL FIELD

This invention relates generally to semiconductor device manufacturingprocesses and, more particularly, to forming a light-emitting diode andseparating a light-emitting diode structure from a growth substrate.

BACKGROUND

Compound semiconductor devices are widely used for optoelectronicapplications. For example, compound semiconductors composed of materialsfrom group III-V are best suitable for light-emitting diodes (LEDs).LEDs are manufactured by forming active regions on a substrate and bydepositing various conductive and semiconductive layers on thesubstrate. The radiative recombination of electron-hole pairs can beused for the generation of electromagnetic radiation (e.g., light) bythe electric current in a p-n junction. In a forward-biased p-n junctionfabricated from a direct band gap material, such as gallium arsenide(GaAs) or gallium nitride (GaN), the recombination of the electron-holepairs injected into the depletion region causes the emission ofelectromagnetic radiation. The electromagnetic radiation may be in thevisible range or may be in a non-visible range. Different colors of LEDsmay be created by using compound materials with different band gaps.

Crystalline compound semiconductor materials, such as GaN, are typicallyformed by epitaxially growing a compound semiconductor layer upon acrystalline substrate of another material, such as a sapphire substratethat has a matching crystallographic plane and is more easily formed.The GaN layer thus formed are processed into electronic oroptoelectronic devices, such as LEDs, based upon the properties of GaN.The compound semiconductor devices are then detached from their growthsubstrate and reattached to other semiconductor or non-semiconductorsubstrates to be integrated with other electronic components for theintended applications.

Various techniques exist in separating a compound semiconductor layerfrom a growth substrate. In one attempt, an epitaxial sacrificial layeris first grown on the substrate. The compound semiconductor layer isthen epitaxially grown on the sacrificial layer. After the compoundsemiconductor layer is processed with the intended devices, it isseparated from its growth substrate by a wet etching process, whichselectively etches away the sacrificial layer, thereby lifting off thecompound semiconductor layer. The free-standing compound semiconductorfilm may be then bonded to other substrates. The compound thin film maybe further processed to integrate the functionalities of the compoundsemiconductor devices and of devices in the other substrate material.

The above existing separating process relies upon a liquid etchantdissolving from the sides of a very thin sacrificial layer between thegrowth substrate and epitaxially formed compound semiconductor film. Theseparating process can be very time-consuming, especially for separatinglarge area films, and are economically unfavorable for manufacturingprocesses of large scale.

In another attempt, an optical process is deployed in lifting offcompound films from a growth substrate. As an example, a GaN film isepitaxially grown on a sapphire substrate. The resultant structure isthen irradiated from the sapphire side with an intense laser beam. Thiswavelength of the laser is within the sapphire bandgap so that theradiation passes through it, but the irradiation wavelength is somewhatoutside of the absorption edge of GaN. As a result, a significantportion of the laser energy is absorbed in the GaN next to theinterface. The intense heating of the GaN separates the gallium fromgaseous nitrogen, thereby separating the GaN thin film from the sapphiresubstrate.

The process, however, suffers various difficulties. As an example, thehigh energy laser radiation may blow away the overlying GaN film, andfracturing of the GaN film often occurs. Moreover, the area of thehigh-energy laser beams is limited, which makes separating large areafilms difficult.

SUMMARY OF THE INVENTION

These and other problems are generally reduced, solved or circumvented,and technical advantages are generally achieved, by embodiments of thepresent invention, which provides a method of forming a light-emittingdiode (LED) device and separating the LED device from a growthsubstrate.

In accordance with one aspect of the present invention, a method offorming a light-emitting diode (LED) comprises providing a firstsubstrate, and forming and patterning a mask layer on the substrate,thereby creating a patterned mask layer. The method also comprisesforming a first contact layer over the patterned mask layer with an airgap between the first contact layer and the patterned mask layer. Themethod further comprises forming an LED structure wherein the firstcontact layer is a contact layer of the LED structure, and forming asecond substrate on the LED structure wherein the second substrate isconductive. The method even further comprises separating the LEDstructure from the first substrate.

In accordance with another aspect of the present invention, a method offorming an LED device comprises providing a first substrate, and formingand patterning a mask layer on a first side of the substrate, therebycreating a patterned mask layer. The method also comprises forming anLED structure over the patterned mask layer with an air bridge betweenthe LED structure and the patterned mask layer, forming a secondsubstrate over the LED structure on an opposing side of the LEDstructure from the first substrate, and detaching the first substratefrom the LED structure through a wet etch process.

In accordance with yet another aspect of the present invention, a methodof forming an LED device comprises forming and patterning a mask layeron a growth substrate, thereby creating a patterned mask layer. Themethod also comprises forming seed regions in one or more openings inthe patterned mask layer, the seed regions protruding from the patternedmask layer. The method further comprises laterally growing from the seedregions until a continuous, first contact layer is formed over thepatterned mask layer with an air bridge between the first contact layerand the patterned mask layer, forming an LED structure wherein the firstcontact layer is a contact layer of the LED structure, and separatingthe LED structure from the growth substrate at the air bridge.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIGS. 1-6 illustrate various intermediate process steps of manufacturinga light-emitting diode device in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A novel method for forming GaN layers for light-emitting diodes (LEDs)and separating the LEDs from a growth substrate is provided. Theintermediate stages of manufacturing preferred embodiments of thepresent invention are illustrated. It should be understood that theshown steps illustrate the inventive aspects of the invention, but otherprocesses may be performed. Throughout the various views andillustrative embodiments of the present invention, like referencenumbers are used to designate like elements.

FIGS. 1-6 illustrate various intermediate process steps of forming anLED on a substrate in accordance with an embodiment of the presentinvention. Referring first to FIG. 1, a wafer 100 is shown including asubstrate 102 with an overlying mask layer 104. The substrate 102 ispreferably a bulk semiconductor substrate, doped or undoped, preferablyhaving a (100) surface orientation. It should be noted that whileembodiments of the present invention are described in the context ofusing a bulk silicon substrate, other substrates may be used. Forexample, silicon-on-insulator (SOI) substrates, sapphire substrates, SiCsubstrates, and the like may also be used. Embodiments of the presentinvention, however, may be particularly suited to silicon substrates dueto the low cost. Furthermore, while a substrate having a (100) surfaceorientation is preferred, substrates having a different surfaceorientation, such as (110) or (111) surface orientations, may also beused.

The mask layer 104 is preferably a hard mask comprising one or moredielectric layers. For example, silicon nitride (SiNx) formed through aprocess such as chemical vapor deposition (CVD) may be used. In anotherembodiment, mask layer 104 may be a silicon dioxide layer formed by, forexample, thermal oxidation or by chemical vapor deposition (CVD)techniques using tetra-ethyl-ortho-silicate (TEOS) and oxygen asprecursor. Alternatively, other dielectric materials, such as siliconoxynitride, or the like may be formed through a process such as CVD forforming mask layer 104. A multi-layer hard mask, such as layers ofsilicon dioxide and silicon nitride, may also be used. The mask layer104 preferably has a thickness of about 50 Å to about 200 Å.

As illustrated in FIGS. 2 a and 2 b, the mask layer 104 (see FIG. 1) issubsequently patterned to form a patterned mask 206 in accordance withan embodiment of the present invention. In an embodiment, the mask layer104 is patterned using photolithography techniques known in the art.Generally, photolithography techniques involve depositing a photoresistmaterial and irradiating the photoresist material in accordance with apattern. Thereafter, the photoresist material is developed to remove aportion of the photoresist material. An etching process is subsequentlyperformed on wafer 100 creating openings 208 in the patterned mask 206.The remaining photoresist material protects the underlying mask layermaterial during the etching process.

After the etching process, a matrix of openings 208 are created in thepatterned mask 206, exposing the underlying substrate 102. Each openingpreferably has a height width w of about 2 μm to about 10 μm, as shownin FIG. 2 b. The spacing between adjacent openings 208 has a distance ofabout 5 μm to about 10 μm. It should be noted that the embodimentillustrated in FIG. 2 b illustrates square openings for illustrativepurposes only. Other embodiments may use any suitable shape, includingrectangles, stripes, circles, ovals, triangles, and/or the like.Furthermore, other embodiments may not be arranged in a matrix formed byrows and columns, but rather may include openings in a patterned,staggered, or the like arrangement. A, preferably GaN, seed 220 a isthen formed in each of the openings 208, as shown in FIG. 2 c.

FIG. 3 illustrates forming a first contact layer 220 formed inaccordance with an embodiment of the present invention. In anembodiment, the first contact layer 220 may be formed of a groupIII-nitride (or other group V element), e.g. GaN, and used as a firstcontact layer of an LED structure as will be discussed in detail below.In an embodiment, the first contact layer 220 includes raised regions(also sometimes referred to as seed regions or seeds) 220 a formed inthe openings 208 and lateral regions 220 b grown over the patterned mask206. The seed regions 220 a are formed in the openings 208 vertically(vertical arrows in FIG. 3), protruding from the patterned mask 206. Theprotruding portions of the seed regions 220 a exhibit crystalline facetson their sidewalls. After forming seed regions 220 a, the lateralregions 220 b are formed by a lateral epitaxial growth of GaN from thenewly shaped crystalline facets on the sidewalls of GaN seed regions,extending horizontally (lateral arrows in FIG. 3) over the patternedmask 206. The lateral growth of GaN 220 b (also sometimes referred to aslateral regions) proceeds over patterned mask 206 to form the firstcontact layer 220 in a continuous form as shown. As a result of thelateral growth process, air bridges 210 are formed between GaN firstcontact layer 220 and the patterned mask 206. In an embodiment, thefirst contact layer 220 above the patterned mask 206 preferably has athickness of about 500 nm to about 3000 nm, and air bridge 210 thusformed has a depth of about 0.3 μm to about 1.0 μM.

In an embodiment where GaN is used to form the first contact layer 220,wafer 100 with the patterned mask 206 may be first processed in aselective metal organic chemical vapor deposition (MOCVD) process usingtrimethylgallium (TMG) and NH₃ as the Ga and N sources to form the GaNseed regions 220 a. After the formation of GaN seed regions 220 a,wafers are continuously processed to form the lateral regions 220 b inthe same MOCVD processing reactor. In another embodiment, forming GaNseed regions 220 a and lateral regions 220 b of the first contact layer220 are performed in separate MOCVD processing chambers. In anembodiment, high temperature MOCVD process is used to form first contactlayer 220 between about 700° C. and about 1100° C. Alternatively, firstcontact layer may be formed in a low-temperature MOCVD process, forexample, between about 300° C. and about 700° C. Other processes, suchas a remote plasma-enhanced chemical vapor deposition (RPCVD),molecular-beam epitaxy (MBE), metal organic vapor phase epitaxy (MOVPE),hydride vapor phase epitaxy (HVPE), liquid phase epitaxy (LPE), or thelike, may also be used to form the first contact layer 220.

In an embodiment, the first contact layer 220 is doped with n-typeimpurity. In other embodiments, the first contact layer 220 may be dopedwith a p-type impurity or substantially un-doped. Alternatively, thefirst contact layer 220 may also include other group-III nitrides, suchas InN, AlN, In_(x)Ga_((1-x))N, Al_(x)In_(y)Ga_((1-x-y))N, or the like,formed with applicable deposition techniques in a similar processdescribed above, which creates the air bridges 210 between the firstcontact layer 220 and the patterned mask 206. Other materials, includingother group V elements instead of nitride, may be also used.

FIG. 4 illustrates completing an LED structure 300 over the substrate102 in which the first contact layer 220 acts as a contact layer for anLED in accordance with an embodiment of the present invention. The LEDstructure 300 may include the first contact layer 220 formed with methoddescribed above, an optional first cladding layer 222, light emittinglayer 224, an optional second cladding layer 228, and a second contactlayer 230.

The optional first cladding layer 222 is formed over the first contactlayer 220. Similar to the first contact layer 220, the first claddinglayer 222 may be formed of a group III-N compound (or other group Velement). In an exemplary embodiment, the first cladding layer 222comprises a group III-N compound having an n-type conductivity (e.g.,n-AlGaN). The formation methods of the first cladding layer 222 may beessentially the same as the method for forming first contact layer 220.

The light-emitting layer (also sometimes referred to as an active layer)224 is formed on first cladding layer 222. The light-emitting layer 224may include a homojunction, heterojunction, single-quantum well (SQW),multiple-quantum well (MQW), or the like structure. In an exemplaryembodiment, the light-emitting layer 224 comprises undoped galliumindium nitride (Ga_(x)In_(y)N_((1-x-y))). In alternative embodiments,light-emitting layer 224 includes other commonly used materials such asAl_(x)In_(y)Ga_((1-x-y))N. In yet other embodiments, light-emittinglayer 224 may be a multiple quantum well including multiple well layers(such as InGaN) and barrier layers (such as GaN) allocated in analternating pattern. Again, the formation methods include MOCVD, MBE,HVPE, LPE, or other applicable CVD methods. The total thickness of thelight-emitting layer 224 is preferably between about 5 nm and about 200nm.

The optional second cladding layer 228 is formed on light-emitting layer224. In an embodiment, the second cladding layer 228 comprises amaterial similar to that of first cladding layer 222, such as AlGaN,except the second cladding layer 228 may be doped to p-type. Theformation method of the second cladding layer 228 may be essentially thesame as the method for forming the first cladding layer 222, excepthaving an opposite type of conductivity.

The second contact layer 230 is formed on the second cladding layer 228.The second contact layer 230 may be formed of essentially the same ordifferent materials, and using similar methods, as the formation offirst contact layer 220, except the conductivity type of the secondcontact layer 230 is opposite to that of the first contact layer 220.

Also shown in FIG. 4 is a reflective layer 250 formed over the top ofthe group III-V LED structure 300. The reflective layer 250 acts toreflect light emitted from the light-emitting layer 224 toward andthrough the second contact layer 230 back toward the first contact layer220, which will act as the light-emitting surface of the LED device asdiscussed below. The reflective layer 250 may comprise a single layer ofreflective metal, e.g., Al, Ag, or the like. Thus, in one embodiment,reflective layer 250 also acts as an electrode providing electricalcontact to a p-type second contact layer 230. The reflective layer 250may comprise a multiple layer structure, such as a distributed Braggreflector, an omni-directional reflector, or the like. The reflectivelayer 250 preferably has a thickness from about 50 Å to about 500 Å. Inother embodiments in which the second contact layer 230 is highlyreflective, the reflective layer 250 may not be necessary.

FIG. 5 illustrates forming a conductive substrate 280 on the LEDstructure 300 in accordance with an embodiment of the present invention.The conductive substrate 280 is formed over the LED structure 300 andthe reflective layer 250, and provides an electrical contact to theconductive reflective layer 250 (and/or the second contact layer 230).The conductive substrate 280 may be formed of any suitable conductivematerial, such as doped silicon, metal, metal alloy, or the like. Theconductive substrate 280 preferably has a thickness greater than about50 μm.

In an embodiment, the conductive substrate 280 is formed byelectroplating. In this embodiment, the wafer is coated with a metal,such as aluminum, nickel, chromium, copper, or the like, in a single ormultiple layer structure.

In another embodiment, the conductive substrate 280 is formed ofsilicon. In this embodiment, a silicon substrate is bonded to thesurface of the reflective layer 250, thereby forming the conductivesubstrate 280 as illustrated in FIG. 5. In an embodiment, the siliconsubstrate that is bonded is preferably a bulk silicon substrate dopedwith ions having a conductivity type the same as the second contactlayer 230 of the LED structure 300. In another embodiment, the bondedsubstrate 280 is pre-processed with one or more through silicon vias(TSVs) 263, which may be coupled to one or more semiconductor devices265 pre-formed in the same or different substrate through one or moremetal trace 264. These semiconductor devices are electrically coupled tothe LED structure 300 after the substrate 280 is bonded.

FIG. 6 illustrates removing the substrate 102 and the patterned mask 206in accordance with an embodiment of the present invention. In oneembodiment, the substrate 102 and the patterned mask 206 may be removedby wet chemical etch processes. In this embodiment, a phosphoric acidsolution of about 120° C. to about 160° C. may be used to etch thesilicon nitride patterned mask 206 from the silicon substrate 102because the phosphoric acid has a high etch selectivity rate of siliconnitride mask 206 to the silicon substrate 102 (and other layers of theLED structure 300). After the removal of the patterned mask 206, thesilicon substrate 102 may be removed by, for example, a wet dip in asolution of hydrofluoric acid, nitric acid, and acetic acid (commonlyreferred to as an HNA solution).

It is noted that, due to the existence of air bridges 210 between theLED structure 300 and the patterned mask layer 206 (FIG. 5), improvedefficiency can be achieved in separating the LED structure from thegrowth substrate 102. Once the wafers are immersed in an etchingsolution, the liquid etchant may reach the entire substrate surfacethrough air bridges 210, and the separating process can be significantlyexpedited. The separated substrate 102 may advantageously be reused,thereby reducing waste and reducing costs. The air bridges 210 alsofacilitates separating large area films from a large growth substrate,thus enhancing manufacturing throughput.

It is further noted that, after separating the LED structure 300 fromthe silicon substrate 102, the first contact layer 220 of the LED devicehas a textured face due to the protruding GaN features (FIG. 6) leftafter the removal of the patterned mask 206 and the growth substrate102. These protruding features may have any suitable shapes dependingupon the etch pattern on the patterned mask 206, such as squares,stripes, rectangles, circles, ovals, triangles, and/or the like. Atextured light-emitting face on an LED device is typically a favorablefeature that is formed to avoid light reflection at the light-emittingface.

Thereafter, processes may be performed to complete the LED device. Forexample, electrical contacts (front-side and/or back-side contacts) maybe formed to the first and second contact layers, passivation layers maybe formed, and the LED device may be diced and packaged.

While the above description assumes that the LED structure has a p-typesurface facing the conductive substrate 280, one of ordinary skill inthe art will appreciate that embodiments of the present invention mayutilize an LED structure such that an n-type surface faces theconductive substrate. In these embodiments, the first contact layer 220and the optional first cladding layer 222 would have a p-typeconductivity, and the second cladding layer 228 and the second contactlayer 230 would have an n-type conductivity.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method of forming a light-emitting diode (LED) device, the methodcomprising: providing a first substrate; forming and patterning a masklayer on a planar surface of the substrate, thereby creating a patternedmask layer; growing a continuous first contact layer over the patternedmask layer with an air gap between the first contact layer and thepatterned mask layer; forming an LED structure, wherein the firstcontact layer is a contact layer of the LED structure; bonding a secondsubstrate on the LED structure; and separating the LED structure fromthe first substrate.
 2. The method of claim 1, wherein the growing thecontinuous first contact layer comprises both a vertical epitaxialgrowth process and a horizontal epitaxial growth process.
 3. The methodof claim 1, further comprising: forming a reflective layer over the LEDstructure before the bonding the second substrate, wherein the bondingthe second substrate is performed so that the reflective layer isdisposed between the first and second substrates.
 4. The method of claim1, wherein the separating the LED structure comprises a wet chemicaletch process.
 5. The method of claim 1, wherein the LED structureincludes a second contact layer having a different type of conductivityfrom the first contact layer.
 6. The method of claim 1, wherein: thefirst substrate includes bulk silicon; and the second substrate includesone of: metal, metal alloy, and doped silicon.
 7. The method of claim 1,wherein the second substrate includes one or more through-silicon-vias.8. A photonic device, comprising: a first group III-V compound layer,wherein the first group III-V compound layer includes a first surfacethat is substantially planar and a second surface opposite the firstsurface, the second surface being non-planar and having a plurality ofprotrusions, wherein the protrusions each have substantially planarsidewalls and a planar end portion; a light-emitting layer disposed overthe planar surface of the first group III-V compound layer; and a secondgroup III-V compound layer disposed over the light-emitting layer in amanner such that the light-emitting layer is disposed between the firstand second group III-V compound layers.
 9. The photonic device of claim8, wherein the protrusions are arranged in a matrix having a pluralityof rows and columns from a top view.
 10. The photonic device of claim 8,wherein the protrusions have shapes that are selected from the groupconsisting of: squares, stripes, rectangles, circles, ovals, andtriangles.
 11. The photonic device of claim 8, wherein the protrusionsprotrude in a direction away from the first surface by a distancegreater than about 50 Angstroms.
 12. The photonic device of claim 8,wherein: the first and second group III-V compound layers have differenttypes of conductivity; and the light-emitting layer includes a multiplequantum well (MQW).
 13. The photonic device of claim 8, furthercomprising a substrate bonded to the second group III-V compound layer,wherein the substrate contains one of: metal, metal alloy, and dopedsilicon.
 14. The photonic device of claim 13, wherein the substrateincludes a through-silicon-via.
 15. A method of fabricatinglight-emitting diode (LED), comprising: forming a mask layer over asubstantially flat surface of a first substrate, the mask layer having aplurality of openings that expose portions of the substrate; forming aplurality of seed regions in the plurality of openings through avertical epitaxial growth process, the seed regions containing a firstgroup III-V compound; uniting the plurality of seed regions through ahorizontal epitaxial growth process, thereby forming a first contactlayer, wherein the first contact layer is formed in a manner such thatgaps exist between the first contact layer and the mask layer disposedbelow; forming a light-emitting layer over the first contact layer;forming a second contact layer over the light-emitting layer, the secondcontact layer containing a second group III-V compound, the first andsecond group III-V compound having different types of conductivity;coupling a second substrate to the second contact layer; and thereafterremoving the first substrate and the mask layer.
 16. The method of claim15, wherein the removing the first substrate is performed through a wetchemical etch process.
 17. The method of claim 15, further comprising:forming a reflective layer over the second contact layer, wherein thereflective layer is disposed between the second contact layer and thesecond substrate.
 18. The method of claim 15, wherein the plurality ofopenings are arranged in a matrix having a plurality of rows and columnsin a top view.
 19. The method of claim 15, wherein the second substratecontains one of: metal, metal alloy, and doped silicon.
 20. The methodof claim 19, wherein the second substrate includes a plurality ofthrough-silicon-vias.